The ability to produce and store hydrogen efficiently, economically and safely, is one of the challenges to be overcome to make hydrogen an economic source of energy. There have been described the limitations in the current commercialization of fuel cells, and internal combustion engines fueled with hydrogen.
The hydrogen storage methodologies span many approaches, including high pressures and cryogenics, but usually focus on chemical compounds that reversibly release H2 upon heating. Hydrogen storage is a topical goal in the development of a hydrogen economy. Most research into hydrogen storage is focused on storing hydrogen in a lightweight, compact manner for mobile applications. Hydrocarbons are stored extensively at the point of use, be it in the gasoline tanks of automobiles. Hydrogen, in comparison, is quite difficult to store or transport with current technology. Hydrogen gas has good energy density by weight, but poor energy density by volume versus hydrocarbons, hence it requires a larger tank to store. Increasing gas pressure would improve the energy density by volume, making for smaller, but not lighter container tanks. Thus, higher compression will mean more energy lost to the compression step.
Alternatively, metal hydrides, with varying degrees of efficiency, can be used as a storage medium for hydrogen. Some are easy-to-fuel liquids at ambient temperature and pressure, others are solids which could be turned into pellets. Proposed hydrides for use in a hydrogen economy include simple hydrides of magnesium or transition metals and complex metal hydrides, typically containing sodium, lithium, or calcium and aluminium or boron. These materials have good energy density by volume, although their energy density by weight is often worse than the leading hydrocarbon fuels. Furthermore, high temperatures are often required to release their hydrogen content. Solid hydride storage is a leading contender for automotive storage. A hydride tank is about three times larger and four times heavier than a gasoline tank holding the same energy. For a standard car, that's about 0.17 m3 of space and 270 kg versus 0.057 m3 and 70 kg. Lithium, the primary constituent by weight of a hydride storage vessel, currently costs over 90 $/kg. Any hydride will need to be recycled or recharged with hydrogen, either on board the automobile or at a recycling plant. A metal-oxide fuel cell, (i.e. zinc-air fuel cell or lithium-air fuel cell), may provide a better use for the added weight, than a hydrogen fuel cell with a metal hydride storage tank. Often hydrides react by combusting rather violently upon exposure to moist air, and are quite toxic to humans in contact with the skin or eyes, hence cumbersome to handle (see borane, lithium aluminum hydride). For this reason, such fuels, despite being proposed and vigorously researched by the space launch industry, have never been used in any actual launch vehicle. Few hydrides provide low reactivity (high safety) and high hydrogen storage densities (above 10% by weight). Leading candidates are sodium borohydride, lithium aluminum hydride and ammonia borane. Sodium borohydride and ammonia borane can be stored as a liquid when mixed with water, but must be stored at very high concentrations to produce desirable hydrogen densities, thus requiring complicated water recycling systems in a fuel cell. As a liquid, sodium borohydride provides the advantage of being able to react directly in a fuel cell, allowing the production of cheaper, more efficient and more powerful fuels cells that do not need platinum catalysts. Recycling sodium borohydride is energy expensive and would require recycling plants. More energy efficient means of recycling sodium borohydride are still experimental. Recycling ammonia borane by any means is still experimental. Hydrogen produced for metal hydride storage must be of a high purity. Contaminants alter the nascent hydride surface and prevent absorption. This limits contaminants to at most 10 ppm oxygen in the hydrogen stream, with carbon monoxide, hydrocarbons and water at very low levels. An alternative to hydrides is to use regular hydrocarbon fuels as the hydrogen carrier. Then a small hydrogen reformer would extract the hydrogen as needed by the fuel cell. However, these reformers are slow to react to changes in demand and add a large incremental cost to the vehicle powertrain. Direct methanol fuel cells do not require a reformer, but provide a lower energy density compared to conventional fuel cells, although this could be counter balanced with the much better energy densities of ethanol and methanol over hydrogen. Alcohol fuel is a renewable resource. Solid-oxide fuel cells can operate on light hydrocarbons such as propane and methane without a reformer, or can run on higher hydrocarbons with only partial reforming, but the high temperature and slow startup time of these fuel cells are problematic for automotive applications. Some other hydrogen carriers strategies including carbon nanotubes, metal-organic frameworks, doped polymers, glass microspheres, phosphonium borate, imidazolium ionic liquids, amine borane complexes have been investigated with moderate results. On the other hand, ammonia has been investigated as a potent hydrogen precursor. Thus, Ammonia (NH3) releases H2 in an appropriate catalytic reformer. Ammonia provides high hydrogen storage densities as a liquid with mild pressurization and cryogenic constraints: It can also be stored as a liquid at room temperature and pressure when mixed with water. Nevertheless, ammonia is a toxic gas at normal temperature and pressure and has a potent odor.
The patent application WO 2008/094840 discloses a method for producing hydrogen from hydrolysis of organosilane compounds in the presence of a sodium hydroxide solution and a catalyst consisting of a substoichiometric amount of an organic amine, notably the n-octylamine and n-propylamine. However, some of the used organosilane compounds such as siloxene are expensive and quite toxic. Furthermore, such compounds often lead to the formation of not environment-friendly by-products of which recycling has not been completely envisioned and appears quite difficult and expensive.
There remains a need for further improvements in efficiency, performance, and cost effectiveness of such clean energy sources, for a variety of applications, such as portable and stationary fuels cells or emissions control system for motor vehicles. There remains a need for improvements which exhibit enhanced efficiency, performance and that are cost effective.
It now has been discovered that by using a phosphorous based catalysts in a basic aqueous solvent, hydrogen could be produced in large amounts, with high yields, in a very short time and with very low production costs. More particularly, hydrogen may be advantageously produced in one step from unexpensive commercially available products. Further, this method can be easily scaled up.
Thus, in one aspect, the invention is directed to a method for producing hydrogen (H2) comprising the steps consisting in:                i) contacting a compound (C) comprising one or more groups Si—H with a phosphorous based catalyst in the presence of a base in water as a solvent, thereby forming hydrogen and a by-product (C1);        wherein said phosphorous based catalyst is selected from:                    a compound of formula X1X2X3P(═O) wherein:X1, X2, X3 are each, independently selected from C1-C6 alkyl, C1-C6 alkoxy, NRaRb, C6-C10 aryl, aralkyl, 5 to 7 membered heteroaryl;wherein said alkyl or aryl groups are optionally substituted by one to three Rc;orX1 and X2 together form with the phosphorous atom to which they are attached a 3 to 10 membered heterocycloalkyl optionally substituted by Rd; and X3 is defined as above; orX3 is -L-P(═O) X1X2, wherein L is C1-C6 alkylene or C6-C10 arylene and X1, X2 are as defined above;Ra and Rb are each independently selected from C1-C6 alkyl, C6-C10 aryl or together form with the phosphorous atom to which they are attached a heterocyclyl optionally substituted by one to three Re;Rc, Rd and Re are each independently selected from Cl, Br, I, F, OH, C1-C6 alkyl, C1-C6 alkoxy, NO2, NH2, CN, COOH;                        a polymer-supported catalyst bearing one or more groups RaRb(P═O)—, RaRb being as defined hereabove;        ii) recovering the obtained hydrogen.        
Preferably, one of X1, X2, X3 is NRaRb.
Preferably, Ra and/or Rb is/are C1-C6 alkyl, or heterocycloakyl, more preferably C1-C6 alkyl.
Preferably, the phosphorous based catalyst is (O═)P(NRaRb)3 
In a particularly preferred embodiment, the phosphorous based catalyst is hexamethylphosphoramide (HMPA).
In a variant, the catalyst is grafted onto a polymer such as (Aminomethyl)polystyrene, also referred to as polystyrene AM-NH2.
The molar ratio of the phosphorous based catalyst relative to compound (C) ranges preferably from 0.01 to 0.5 equivalents, most preferably from 0.01 to 0.1 equivalents.
Preferably, the base is a mineral base, notably an alkaline or alkaline-earth metal hydroxide, such as potassium hydroxide or sodium hydroxide, the sodium hydroxide being particularly preferred.
Preferably, the hydroxide aqueous solution has a concentration ranging from 5 to 40% in water (weight/weight).
The temperature of the reaction in step a) of the method according to the invention may vary in a wide range, and may range notably from 0 to 200° C. More preferably, the temperature ranges from 15 to 30° C. and is most preferably of about 20° C.
Preferably, the compound (C) comprises at least two groups Si—H.
Preferably, the compound (C) comprises one or more monomer units of formula (A):
wherein:                R is a bond, C1-C6 alkylene, (C1-C4 alkylene)m-Z—(C1-C4 alkylene)q;        Z is O, NR10, S(O)y, CR10═CR10, C≡C, C6-C10 arylene, 5-10 membered heteroarylene, or C3-C6 cycloalkylene;        R1, R2 are each independently selected from H, halogen, C1-C10 alkyl, C3-C10 cycloalkyl, C6-C12 aryl, aralkyl, 5 to 10-membered heteroaryl, OR3, NR4R5, SiR6R7R8, wherein said aryl groups are optionally substituted by one to three R9 groups;        R3 is H, C1-C6 alkyl, C6-C10 aryl, aralkyl;        R4, R5 are each independently selected from H, C1-C6 alkyl, C6-C10 aryl, aralkyl;        R6, R7, R8 are each independently selected from H, OR3, C1-C6 alkyl, C6-C10 aryl, aralkyl;        R9 is selected from halogen, C1-C10 alkyl, OR10, NO2, NR11R12, CN, C(═O)R10, C(═O)OR10, S(═O)CH3, wherein said alkyl group is optionally substituted by one or more halogen;        R10 is H or C1-C3 alkyl;        R11, R12 are each independently selected from H, or C1-C10 alkyl;        m, q are 0 or 1;        y is 0, 1 or 2;        n, p are integers each representing the number of repeating units, with                    n being superior or equal than 1, and            p being 0 or 1.                        
In a preferred embodiment, p is 0.
In a preferred aspect of the invention, the compound (C) comprises one or more monomer unit of formula (Ia):

Preferably, the compound comprising a monomer unit of formula (Ia) is tetrasilylmethane ((H3Si)4C), phenylsilane (PhSiH3), or N,N-diethyl-1,1-dimethylsilylamine ((Et)2N—SiH(CH3)2), tetrasilylmethane and phenylsilane being particularly preferred.
In a still further preferred embodiment, p is 1.
Preferably, R is a bond or C1-C6 alkylene, notably —CH2—CH2—. Alternatively, R is Z, with Z being O or NR10, notably NH.
Preferably, the monomer unit is of formula (Ib):

Preferably, the compound (C) comprising a monomer unit of formula (Ib) is tetramethyldisiloxane ((CH3)2HSi—O—SiH(CH3)2), 1,1,3,3-tetramethyldisilazane ((CH3)2HSi—NH—SiH(CH3)2), 1,4-disilabutane (H3Si(CH2)2SiH3), or tetramethyl-disilane ((CH3)2HSi—SiH(CH3)2), 1,4-disilabutane being particularly preferred.
Phenylsilane and disilabutane are advantageously commercially available, easy to handle, stable to air and moisture, and can be stored for long periods of time without loss of activity. Finally, tetrasilylmethane, phenylsilane and 1,4-disilabutane have both revealed to be hydrogen carriers with a high hydrogen storage density.
In a particular embodiment, the method of the invention further comprises a step c) of recycling the obtained by-product (C1).
Thus, the method of the invention may further comprises two subsequent steps, after step a):                c) contacting the by-product (C1) with an acyl halide;        d) contacting the obtained product with a metal hydride, thereby regenerating compound (C).        
The acyl halide may be notably CH3C(═O)Cl. The metal hydride may be notably an aluminum hydride such as LiAlH4.
As an example, recycling the silylated derivative may be performed according to the following scheme:

More generally, the invention relates to a method comprising:
i) producing hydrogen from a compound (C); and
ii) recycling the obtained by-product (C1) of step i).
The hydrogen obtained by the method of the invention is recovered, either for storage or for use to produce energy.
In another aspect, the invention relates to a composition comprising a compound (C), a phosphorous based catalyst, a base and water as a solvent as described hereabove.
Particularly preferred compositions are those comprising tetrasilylmethane, phenylsilane or 1,4-disilabutane in combination with a catalytic amount of a phosphorous catalyst and a 10% potassium hydroxide solution.
As a further aspect, the invention relates to the use of a composition according to the invention for producing hydrogen.
In particular, the compositions, or compound (C) in the presence of a catalytic amount of a phosphorous catalyst and a 10% potassium hydroxide solution may be used as a fuel, a propellant or a precursor thereof. As an example, they may be used as a fuel in a fuel cell, in an engine as a NOx reducing agent or as a supplementary fuel or for any other consuming device. As another example, they may be used in a battery.
In an additional aspect, the invention relates to a device for producing hydrogen according to the method hereabove described, said device comprising a reaction chamber comprising:                i. A reaction mixture inlet, said mixture comprising a compound (C), a base in water as a solvent;        ii. an hydrogen outlet;        iii. a by-product collector; and        iv. a surface intended to be in contact with said mixture, coated with a polymer supported catalyst as described hereabove.        
Preferably, the device of the invention further comprises a mixing chamber for mixing the compound (C) with the base in water as a solvent, wherein the mixing chamber is connected to the reaction chamber.
Preferably, the device of the invention further comprises a by-product collection chamber, wherein the collection chamber is connected to the reaction chamber.
Preferably, the device of the invention further comprises a second chamber comprising:                v. an outer envelope;        vi. an internal wall separating said chamber into two distinct compartments, namely:                    1. a first compartment containing the reaction mixture to be introduced in the reaction chamber; and            2. a second compartment containing the by-product (C1) collected from the reaction chamber;            3. the first and second compartment being each connected to the reaction chamber;                         and        vii. means for moving the internal wall relative to the outer envelope, so as to make the respective volumes of each compartment to vary.        